Intentionally Upsized 150 KVA Transformer?

[Rep]

I have a client who wishes to use a 200 amp 3 phase 50 hp motor and associated equipment. It comes out at 208 amps of capacity and so I would normally put in a 75 kva transformer to kick his 208V up to ~480. No problems so far. However, he anticipates putting in a second 50 hp motor and he has a 150 kva transformer already and he wishes to avoid buying a 75 kva if he doesn't have to.

Can we size everything to the 150 KVA, put in 200 amp fuses for now (resize them when he adds the new 50 hp motor) and call it a day? I don't have a good explanation as to why a 150 kva will not *work* but I would like to be able to tell him exactly why it would or would not be a bad idea. Safety wise, I think he is fine. Everything is properly sized to handle the load. I worry though about power factor and other associated losses.

Any thoughts?

 

[charlie b]

If you have a calculated load of less than 75 KVA, there is no reason you cannot use a transformer rated at 150 KVA or even larger. If you size the conductors for the 150 KVA ratings, you don't need to fuse it for a 75 KVA rating. But I don't understand this part:

. . . I would normally put in a 75 kva transformer to kick his 208V up to ~480.

What is the voltage supply, and are you suggesting changing it?

 

[dkarst]

a 200 amp 3 phase 50 hp motor

I agree there appears to be some bad math... a 200 amp, 460V motor as seems to be implied is way larger than 50HP or there is something else going on?

 

[Rep]

Perhaps I should clarify.

Input is 200A 3 phase 208V. The equipment needs about 200 amp 208V to run everything, including the 50 hp motor.

It is a 50 hp 480V motor plus some other few hp motors to recirculate some fluids plus run some servos, etc. It normally calls for a 75 KVA transformer to run everything and allow for FLA and start current with a bit of headroom for motor starts, surges, etc. The specs call for this 50 hp motor and associated equipment to be fed by a 200 amp main, transformed up to 480V. The total running amps should be around 120 - 140 @ 208V or so under running conditions. So I don't think there is any issue there.

That all means a 75 KVA transformer is needed. The question really is what effect, if any, would there be if we ran it through a 150 KVA instead because the customer already has the equipment. Would there be losses due to power harmonics, load factor, etc or is it more akin to wires - the larger the better, (less heat loss, etc) they just cost more.

 

[rbalex]

As Charlie noted before, if you size everything upstream for the 150kVA transformer, you won't need to do anything to that part of the system when the new load is added. Assuming a judicious selection for the secondary distribution equipment, your future modifications would be limited to new motor branch circuit and control wiring and the appropriate OCPDs and controls.

You may experience a very modest increase in total losses with the initial installation between an "under-loaded" 150kVA and 75kVA transformer because the "no-load" losses are higher for larger transformers in common designs and sizes; but in the long run, your client will be ahead.

 

[Rep]

When you say "very modest" - can you quantify that? Are we talking about a 1% or a 10% change? Some people say "it doesn't matter" and others tell me that you have losses if you don't properly size.

 

[Rep]

Research continues........

http://www.copper.org/Applications/electrical/energy/trans_efficiency.html

Manufacturer B 75 kVA*
High-Efficiency (Copper) High-Efficiency (Copper)
Load Factor 50% 75%
Efficiency 98.61% 98.38%
Temp. Rise
(100% load) 80? C 80? C
Core Loss 0.21 kW 0.21 kW
Cond. Loss 0.32 kW 0.72 kW
Total Loss 0.53 kW 0.93 kW

Which seems to say that at 50% load factor (so the 150 KVA would be 50% loaded, the efficiency would be 98.61% and at 75% load factor, the efficiency would be 98.38%. So it seems to me that using a larger transformer would actually save (very slight) operating costs due to lower heat losses! In this case, going from 50% to 75% load factor would induce losses of only 400 watts, or stated another way, would mean that using a 150 KVA transformer at 50% load would "save" about 400 watts due to heat loss over a "properly sized" 75 KVA transformer, assuming a standard 80C transformer.

Or did I miss something?

 

[rbalex]

Technically, this can only be answered when we are talking ?apples and apples;? i.e., when the only difference between the two transformers is capacity (kVA) and all other relevant design factors are exactly the same. Same design type, primary & secondary voltage, winding material, ?k-factor,? etc. Depending on the design, a larger transformer of one design may have lower no-load losses than a smaller transformer of a different design. In a few rare cases, a larger transformer of the same design may conceivably have better no-load losses because the core steel is laid out differently (which means it isn't really the same design.)

You will always have no-load losses as long as the transformer is energized. They are roughly constant per a given transformer and larger transformers of the same design generally have higher losses since they have more core steel to magnetize.

Load losses basically increase with the square of the load current since they are basically heating the winding conductors. In this case, larger transformers of the same type have a slight advantage since they will typically have larger conductors.

Without knowing the specifics of both the transformers in question it?s impossible to know the ?break even point.? What can be said is that the smaller transformer can never have the same ultimate throughput of the larger transformer ? no matter how efficient either of them is.

 

[Rep]

Yes, as you propose, I am trying to confine this to an apples to apples type of ideal situation.

Assume the same degree rise, voltage, winding type, material, etc.

Assuming a 75 kva transformer cost the same as a 150 kva, which was alike in all other ways, would it make more sense to run a 75 kva @ 100% or a 150 kva @ 50% and preferably, how would one calculate such a difference? I am trying to prepare some actual numbers.

 

[skeshesh]

In my opinion, the answer is there's no way to easily calculate it - much easier to contact mfg or jump on their website.

Basically though, the factors you want to look at are hysteresis effects, core loses, and eddy current losses. I really think a correct engineering approach is to look at proper data from the mfg. as I would bet there's no way you can calculate the effeciency of the xform better than the testing outcome charts they got. So find the effeciency curve, then calculate your load then compare whatever load you got (45% load at 150kVA vs. 90% load at 75kVA) to see which one is more effecient. Then you got to make an engineering judgement call based on your clients needs, system needs (sometimes the client insists system will probably not grow, but just one look at the single like and you can tell they'll need more power down the line), etc. to judge the merits of having system expansion readily available vs. saving a few bux a year.

 

[rbalex]

Simply comparing the losses of a proposed 75kVA transformer at 100% load to the losses of the 150kVA transformer at 50% load should be fairly straightforward and close enough to make a sound judgment call.

Essentially though, it would be hard to justify the 75kVA transformer, if there were a reasonable chance that the additional load would be added.

 

[steve066]

What were you planning on doing on the secondary? Feeding a main circuit breaker panelboard, with separate branch breakers for the two 50 HP loads?

Or just feeding a single circuit breaker or disconnect, and adding a second one later?

Either way, I think you should be OK.

Steve

 

[kbsparky]

.....Input is 200A 3 phase 208V......

I hope you have at least double that available, or else you ain't got enough input Amps to handle a 150 kVA unit.

 

[Rep]

I hope you have at least double that available, or else you ain't got enough input Amps to handle a 150 kVA unit.

Agreed. We would bring in another 200 amp 3 phase to service the transformer. I was thinking parallel feed but haven't looked at the books yet. That or just upsize the wire and reroute the existing 200 amp to a separate breaker.